The invention pertains to a process and apparatus for processing a photosensitive element using a thermal process without the use of solvents to form a relief pattern and, in particular, to a process and apparatus for thermal development of the photosensitive element to form a flexographic printing plate.
Burg et al. in U.S. Pat. No. 3,060,023 describe a process for transferring images from photopolymerized image-bearing elements to a receptor surface using a dry thermal process. A film support with a photopolymerizable stratum is exposed to an image to provide exposed areas that do not melt and xe2x80x9cunderexposedxe2x80x9d areas that can melt and transfer to an image receptive element. In numerous examples, a paper image receptive element is placed in contact with the stratum surface on a film support and heat is applied by a hot flat heating element pressing against the film support to heat the assemblage. In some examples, an unheated assemblage is passed between two rollers, one of which is heated. The paper is stripped from the stratum while still warm, and the transfer of the unexposed material of the stratum to the paper element is effected. Multiple copies of the image on the paper can be obtained by repeating the thermal transfer process. In a few examples, the thermal transfer is accomplished by heating the paper and pressing it into intimate contact with the stratum. It is suggested without further explanation, that xe2x80x9c. . . heat can be applied at any stage of the process prior to the separation step to either or both elements provided the transfer temperatures correspond to at least the softening temperature of the photopolymerizable stratum. Heat can be applied by means well known in the art, e.g., rollers, flat or curved heating surfaces or platens, radiant sources, e.g., heating lamps, etc.xe2x80x9d. Heating times varied in the examples from 0.5 seconds to 10 seconds. Typical film thicknesses are 1 mil to 4 mils and stratum thicknesses are 0.5 mil to 4 mils. In a few examples, it is disclosed that the film support, with a relief image in the stratum, could be further treated with actinic light and used for printing in a rotary press to make copies.
U.S. Pat. No. 5,175,072 issued to Martens describes a thermal process for manufacturing a flexographic printing plate by providing a radiation hardenable composition as a layer on a flexible substrate, imagewise irradiating the composition to harden the composition in irradiated areas, heating the composition layer to a temperature of between 40xc2x0 C. and 200xc2x0 C. to soften the unirradiated area, contacting the composition layer with an absorbent layer which can absorb the softened unirradiated area which is able to flow into the absorbent layer, and removing the absorbent layer, thereby removing the absorbed flowable composition from the unirradiated areas of the composition on the flexible substrate. This leaves behind on the substrate a raised relief structure of irradiated, hardened composition that represents the irradiated image. This relief area becomes the ink receptive surface that both receives ink from an inking roll in a printing process and transfers the ink to the printing substrate during the printing operation. The process can also utilize a predevelopment step that can improve the adhesion of the irradiated, hardened composition to the flexible substrate by first developing a xe2x80x9cfloorxe2x80x9d of hardened composition on the substrate. This is accomplished by transmitting ionizing radiation through the flexible substrate to polymerize a layer of composition adjacent the flexible substrate. This layer is not removed during the imagewise development and removal of the unirradiated composition.
During the heating process, a laminate comprising the flexible substrate, the composition layer, and the absorbent layer are heated by placing them on a heated platen with the flexible substrate, a polyester film, in contact with the platen. The absorbent layer comprises a non-woven nylon web. After a few seconds of warm-up time to allow the laminate to equilibrate with the platen temperature of about 135xc2x0 C., the laminate is passed between two heated, rubber covered, nip rolls moving in a counter-rotating speed at about 30 cm/minute. The rolls are gapped apart so as to lightly compress the laminate as it is introduced into the gap. As the laminate exits the nip rolls, the absorbent layer is lifted from the heated composition surface with steady tension. The unirradiated areas of the composition are removed via absorption into the non-woven web. The heating and pressing steps are repeated several times until at least 75% of the unirradiated composition is removed from the unirradiated areas. It is desireable that the flexible substrate and the irradiated, hardened composition is stable at the elevated temperatures required to remove the unirradiated composition so that the flexible substrate and hardened composition are not distorted by more than 2% in any surface dimension.
U.S. Pat. No. 5,279,697 issued to Peterson et al. describes an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element. The element comprising the flexible film substrate and composition layer, with irradiated and unirradiated areas, is mounted on a preheating drum that is heated with an electrically heated blanket mounted on an inner surface of a main wall of the drum. The heat must travel through the wall of the drum and through the flexible substrate to preheat the composition layer to a temperature near the melt point of the unirradiated area.
A continuous sheet of absorbent layer of non-woven nylon web is pulled from a supply roll and passed over a hot roll to heat the web. The hot roll is urged toward the preheating drum, thereby pressing the heated web against the preheated composition layer of the printing element on the preheating drum. The heat in the absorbent web is transferred to the printing element upon contact so the temperature of the flexible film is raised to a temperature sufficient to enable the unirradiated portions of the composition layer to liquefy and be absorbed into the absorbent layer of non-woven web. The hot roll may be heated by an electrical core heater or by other means that might use steam, oil, hot air, or the like to provide a temperature sufficient to melt a portion of the composition on the flexible film. As the preheating drum and hot roll rotate in contact together, the web is pressed against the printing element to absorb the liquefied unirradiated composition and is then pulled away from the printing element, thereby separating the absorbed composition from the printing element. The web is transported away from the hot roll and is rewound for waste removal or recycling. Several cycles of passing the printing element past the hot roll are repeated to progressively remove most of the unirradiated composition from the printing element. When the unirradiated composition is removed, a printing plate with a raised relief of hardened irradiated area results that is suitable for the printing process. In some cases a final irradiation of the printing element is carried out to harden any residual unirradiated composition.
In the methods and apparatuses disclosed above to Burg et al., Martens, and Peterson et al., the common practice is to heat the film substrate to a temperature near the melting point of the unirradiated composition. However, heating the film substrate can undesirably distort the substrate to the extent that it may affect the quality of the relief area of the printing plate that becomes apparent during printing. It is common that the temperature of the film substrate gradually increases with each repeat of the heating and absorbing cycle, so by the last cycle, the film substrate is almost at the melt point of the unirradiated composition. The problem presented by substrate distortion is that for a three or four color printing process, three or four plates must be made with images that register exactly for the different colors to come together and produce an accurate final image. If one of the plates distorts in one direction and the other distorts in another direction, the images will not register correctly to produce a quality color print. It is common that the substrates have shrinkage characteristics when heated that are non-uniform in the x and y planar directions due to manufacturing procedures. Typically, the substrate shrinks more in the machine direction of manufacture than in the cross-machine direction. It is believed that a substrate shrinkage of 0.1% is the most allowable at the temperatures the substrate sees during plate development; and preferably, the shrinkage should be less than 0.02% for high quality multi-plate color printing. The substrate shrinkage must also be compatible with the composition coated onto the substrate so wrinkles in the completed photosensitive element do not appear. It is difficult to provide a substrate that has low shrinkage at the temperatures required for thermal imaging of a photosensitive element.
In the method of Burg et al., the film and stratum thicknesses are generally too thin for robust printing plates and there is no method or apparatus taught for rapidly producing a thick, robust, printing plate suitable for commercial application. Where Burg et al. heats only the paper, the process removes only a small quantity of stratum necessary to create an image on the paper.
There is a need for an improved process and apparatus of the type described by Peterson et al. that rapidly heats a substantial thickness of the composition layer to the required temperature while limiting unnecessary heating of the flexible film substrate. There is a need to control accumulated heating of the flexible film substrate with each cycle of heating and absorbing.
The invention is a method of heating a composition layer on a flexible substrate to the melt temperature of an unirradiated area of the composition layer and maintaining the flexible substrate at a temperature below the melt temperature of an unirradiated area of the composition layer while pressing a heated absorbent layer against the still hot composition layer, and repeating these steps for multiple cycles. A further embodiment of the method includes a step of cooling the flexible substrate and layer laminate to maintain the flexible substrate at the desired temperature below that of the heated composition layer.
The invention is an apparatus for forming a relief pattern and, in particular, a flexographic printing plate, from a photosensitive element comprising a flexible substrate having an exterior surface and an interior surface, and a composition layer on the substrate capable of being partially liquefied, the composition layer having an exterior surface and an interior surface, with the composition layer and flexible substrate joined at their respective interior surfaces, comprising:
a roller mounted for rotation in a first frame portion for supplying an absorbent material to the exterior surface of the composition layer;
a drum mounted for rotation in a second frame portion with means for supporting the photosensitive element on an outer circumferential surface of the drum with the exterior surface of the flexible substrate contacting said outer surface, the drum positioned for delivering the photosensitive element to the absorbent material, wherein at least one of the first and second frame portions are movable relative to the other;
first heating means for applying heat to the exterior surface of the composition layer on the drum adjacent where the absorbent material contacts the layer at the roller, the first heating means adapted to heat the exterior surface of the layer to a temperature T1 which is equal to or greater than a temperature T2 sufficient to cause a portion of the layer to liquefy, while maintaining the exterior surface of the flexible substrate at a temperature T3 at least 20xc2x0 F. below temperature T1;
second heating means for heating the roller to a temperature capable of heating the exterior surface of the composition layer to a temperature T4 which is equal to or greater than temperature T2 while the absorbent material is contacting the exterior surface of the layer and while maintaining the exterior surface of the flexible substrate at the temperature T3 which is at least 20xc2x0 F. below temperature T4;
pressure means for causing the photosensitive element and the absorbent material to come into contact between the drum and the roller at a pressure sufficient for at least a portion of the liquefied material of the composition layer to be absorbed by the absorbent material; and
separation means for separating the photosensitive element from the absorbent material.
A further embodiment of the apparatus includes a forced cooling means for cooling the photosensitive element where the element is separated from the absorbent material.